With the passage of time, there are increased numbers of commercial applications for single crystal structures formed of a large number of elements and compounds.
The VGF and VB methods of crystal growth enjoy commercial popularity because of the basic simplicity of the apparatus and their ability to economically and consistently grow relatively defect free single crystal structures of moderate size.
The VGF and VB methods are extensively used to grow crystals of both III-V and II-VI compounds. By way of example, it is common practice to grow Gallium Arsenide (GaAs) crystals having a two inch diameter and a length of several inches.
The emergence of large scale integrated (LSI) devices, very large scale integrated (VLSI) devices; and the commercial development of high volume crystal wafers all demand both larger diameter and longer crystals which can be economically grown in a reproducible manner.
While there has been some success in growing larger diameter crystal structures by VGF and VB methods, there is a ever present tendency to develop twinning and other polycrystaline structures. Each failed attempt is costly of time, manpower and money.
The apparatus for practicing the VGF and VB methods, in general terms, comprises: a crucible in which the crystal is grown; and apparatus for controlled heating of the contents of the crucible in a desired heat pattern.
The crucible is held in a fixed position in the heating apparatus and the heat pattern is controlled in Vertical Gradient Freeze (VGF) apparatus; and the crucible is moved vertically through the heating apparatus to achieve the desired heat pattern in Vertical Bridgman (VB) apparatus.
A crucible is composed of a material which does not react with the process materials; and the wall thickness is selected to promote the desired mechanical strength and heat flow.
A typical prior art circular crucible comprises: a seed well, a transition region and a major growth region. The seed well may have a diameter in the order of 0.25 inches and a length in the order of 1.5 inches. The transition region is a truncated cone which joins the seed well at the smaller diameter end; and joins the major growth region at the larger diameter end. Although cones of a wide range of included angles have been employed as transition regions, a transition region having an included angle of 90 degrees is a compromise value which is widely used. The dimensions of the major growth region are tailored to the desired size of the end product. The major growth region and the seed well may taper outwardly in the order of one degree to facilitate removal of the crystal from the crucible or they may have straight walls.
It is well known that the risk of defect nucleation is higher in the transition region than in the major growth region of the crucible. The typical defects which are encountered are: high dislocation clusters, lineages, poly-crystal nuclei, and twin formation. Once such defects are formed in the transition region, the defect usually migrates into the body of the crystal which renders at least part of the product useless.